1. Field of the Invention
The present invention relates generally to a method of forming patterned nanostructured titania (NST) features, and in particular for forming NST features for various electronic applications such as chemical sensing, wear-resistant electrical contacts and photovoltaics.
2. Description of the Related Art
Patterned nanoporous titania (NST) thin films are well known in many prior reports. For instance, NST has been used in chemical detection systems, catalysis and solar energy conversion. In one widely used conventional method (called the sol-gel method), patterned features of TiO2 are made by the hydrolysis and condensation of metal alkoxide precursors. The metal alkoxide precursors, in liquid form, are deposited on a substrate and subsequently decomposed to form a titania gel film. Various techniques have been used to pattern the TiO2 film such as reactive ion etching, embossing and laser trimming. It is noted that except reactive ion etching, other techniques used with this method are not compatible with high-volume semiconductor manufacturing processes. Patterning of the TiO2 film is done either before or after an annealing step in which the film is heated at elevated temperature to convert the amorphous titania gel into crystalline TiO2. In addition, special precautions are usually required to ensure crack formation does not occur and carbon, from the organic precursors, is not incorporated in or on the nanostructured titania features.
In another known method, paste containing titanium dioxide powders is screen printed at desired locations on components using hard mask; example metal mask. The use of hard mask means that accurate alignment techniques are needed to deposit the paste at desired locations on the substrate. Also the probability of adjacent paste developing bridges increases as the pitch decreases, thus lowering process yield.
In yet another known method, self-assembled monolayers (SAMs) are selectively deposited at locations on which TiO2 pads are desired. Selective deposition of TiO2 films occurs by interactions of functional group of the self-assembled monolayers with Ti-containing precursors. Because surface coverage of SAM is, in most cases, not high, this method results in poor yield especially when large-area substrates are used. In addition, relatively long time—up to a few hours—is required for deposition of TiO2 using this method. In addition, edge acuity, or the ratio of standard deviation to the mean of a sample of pattern widths, of titania features formed using this technique is relatively poor, namely, about 2.1% in the state of the art. This latter problem makes it extremely difficult to use this method to form submicrometer features with very small separation.
With the persistent effort to trim cost and the ever-pervading trend of miniaturizing electronic components, the electronics industry is constantly moving towards using larger diameter wafers to increase yield and hence drive cost down, and making features on integrated circuits smaller and closer to each other. It is, therefore, becoming increasingly difficult to form miniaturized features of porous TiO2 in integrated circuitry.
Formation of porous titania by reacting aqueous hydrogen peroxide (aq. H2O2) solution with thick Ti sheets, Ti powder and unpatterned Ti films is already known in prior art scientific literature. Similarly, formation of a porous titania layer by reacting Ti with aqueous NaOH solutions had been reported. However, in all these cases, titania layers formed by reacting aqueous hydrogen peroxide have high crack density and delaminated extensively from the underlying substrate making them unsuitable for applications in integrated microelectronic circuits.
It can be seen, then, that there is a need in the art for titania layers in microelectronic applications. It can also be seen that there is a need in the art for titania layers that are easily produced that do not have high crack densities and do not delaminate from the underlying substrate.